Conventional methods set an air ratio of the fuel and of the combustion air supplied separately according to predetermined values for the operating state of the engine. The actuators provided for this purpose are acted on by a control unit. The control unit determines the quantity of the fuel, leading to the desired operating load of the diesel engine. Often, a substoichiometric fuel/air ratio is set, and the diesel engine is to this extent operated with a lean mix. A set value for an operating variable of the diesel engine is predetermined in the control unit as a criterion for switching to a special operating mode. At this set value, a defined fuel/air ratio is set according to stipulated values for this special operating mode. This may be the case, for example, if there is a device for exhaust-gas aftertreatment, such as a storage catalytic converter, which is desorbed periodically. For this purpose, the special operating mode creates the required desorption atmosphere by setting the fuel/air ratio. DE 197 53 718 C1 discloses such a method, which evaluates certain criteria in order to switch between two different combustion processes. When the switching criterion is recorded, the engine is switched to the special operating mode in order to regenerate a storage catalytic converter.
DE 199 39 988 A1 also discloses a method which switches to a special operating mode of the diesel engine when a defined switching criterion is present to regenerate a storage catalytic converter with regard to the nitrogen oxide salts. For this purpose, in the regeneration mode, an exhaust gas which has a reducing action on the catalytic converter is generated in the special operating mode.
DE 197 50 226 C1 discloses an engine control system which controls the operation of a diesel engine as a function of characteristic diagrams. In this case, separate characteristic diagrams for an operating mode with lean combustion and an operating mode with fuel-rich combustion are stored in an engine management unit. The duration of the regeneration of the device for exhaust-gas aftertreatment is very long in the known methods. Also, the driver of a motor vehicle driven by the engine can often feel the switching to the special operating mode and back, which is undesirable.
It is an object of the present invention to provide a method for operating a diesel engine, in which the operating conditions are improved in the special operating mode.
According to the invention, in the special operating mode, there is at least one afterinjection of fuel, which is separated in time from the main injection. The time of the afterinjection is set so late in the cycle that the fuel injected in the afterinjection is burnt together with the fuel which is already burning. In this way, an exhaust gas with a high temperature and enthalpy is formed. And it is possible both to promote the regeneration of particle filters, in which superstoichiometric operation is provided for, and to achieve efficient sulphur regeneration of NOx storage catalytic converters in substoichiometric mode. The method according to the invention creates exhaust gases which have a reducing action on catalytic converter surfaces both during superstoichiometric combustion, i.e. with a rich fuel/air mix, and during substoichiometric combustion with a lean fuel/air mix. The fuel is introduced in a plurality of partial injections. For example, the fuel, which is delivered prior to the afterinjection which also burns, can be metered to a plurality of main injections or also any desired number of preinjections. The afterinjection quantity which burns as well can advantageously also be metered in a plurality of afterinjections.
It is particularly advantageous for the fuel in the afterinjection which is also burnt to be metered according to the operating state of the engine, so that ultimately the operating load of the engine is established by setting the afterinjection quantity. It is expediently also possible for the injection time of the afterinjection to be set by the control unit according to the operating state. The partial injections which take place before the afterinjection which is also burnt, with an early combustion position, represent a shaping of the injection profile at which the afterinjection can be matched to the engine operation in terms of quantity and type.
In an advantageous configuration of the invention, when switching into the special operating mode and back, the control unit increases the quantity of fuel in the afterinjection which is also burnt while, at the same time, reducing the main injection quantity of subsequent cycles by a corresponding amount, until the intended afterinjection quantity is reached. Accordingly, when switching back to the standard operating state, the afterinjection quantity is reduced with a corresponding increase in the main injection quantity. It is thereby possible to produce a transition from the formation of a lean mix with fuel/air ratios λ<1 to fuel-rich combustion with fuel/air ratios λ>1. The transition from lean to rich combustion during the switching phase can be carried out with continuously or discontinuously varied quantities in the main injection and the matched afterinjection which is also burnt. An indexed torque of the diesel engine, which is kept constant during the switching phase, is used as a control variable for the change in the injection quantities. In this way, the switching or switching back of the operating states takes place without any jerking whatsoever and cannot be felt by the driver of a motor vehicle.
In order, in the special operating mode, to create the exhaust-gas composition which is required for regeneration of a catalytic converter, it may be expedient to add fuel to the exhaust gas. The added fuel is cracked in the exhaust-gas atmosphere, and the hydrocarbon radicals formed contribute to a reduction in the exhaust-gas emissions from the diesel engine. Fuel may be added with one or more additional afterinjections which take place following the afterinjections which are also burnt in accordance with the invention. However, it is optionally also possible to provide an injection of fuel into the exhaust pipe of the diesel engine, which can be metered independently of the engine operation in terms of quantity and time.
The method according to the invention makes it possible to increase the efficiency of the regeneration of NOx adsorber systems, which under certain conditions (storage capacity, active temperature window) store the nitrogen oxides from internal combustion engines during lean combustion (superstoichiometric combustion with λ>1 and accordingly residual oxygen in the exhaust gas). The method according to the invention quickly provides oxygen-free exhaust gas (λ<1) with the maximum possible reducing-agent content, which is required in order to regenerate NOx adsorber systems of this type, i.e. to desorb NOx and, at the same time, convert NOx into nitrogen N2. The afterinjection fuel which is also burnt likewise quickly results in high temperatures in the exhaust gas, which are used to remove deposits of sulphur compounds, which act as a catalyst poison. The high temperatures also allow more rapid regeneration of particle filters. After a cold start, known catalytic converters require a certain time before they reach their starting temperature. The use of the combustion method according to the invention allows this time to be shortened compared to known methods, on account of the significantly higher exhaust-gas temperature, with the result that a considerably lower emission of exhaust gases is achieved while the diesel engine is warming up.
During the setting or controlling of the fuel/air ratio, it is expediently also possible to take account of the quantity of recirculated exhaust gas which is admixed with the fresh air. The control unit drives an exhaust-gas recirculation valve as an actuator of the control arrangement. In a refinement of the invention, the control unit, in addition to setting the injection parameters, also controls the pressure of the fresh air which is supplied, expediently with throttle devices in the intake region and a turbocharging device in the exhaust region of the diesel engine. Alternatively, the pressure control may be effected by the exhaust-gas recirculation valve and the turbocharging device as actuators.
Furthermore, the control unit advantageously controls the mass throughput of fresh air, using the exhaust-gas recirculation valve as actuator in accordance with a measurement signal from an air mass flowmeter. The mass throughput can also be controlled by throttle valves as actuators. In this case, the control unit coordinates all the actuators and continuously tracks the actuating movements as a function of the recorded control deviations from the set value throughout the entire operating range of the diesel engine. The method according to the invention is implemented even in steady operating states of the diesel engine by corresponding preset values on characteristic diagrams. To set a transition which is neutral in terms of load, the air path is continuously adjusted, specifically the induction pipe pressure and the mass flow of fresh air are regulated, before the injection measures are triggered when switching the operating states, i.e. before the injection quantity of fuel in the afterinjection which is also burnt is altered.
The invention is explained in more detail below with reference to an exemplary embodiment.